Synchronous Pulse Width Modulation (PWM) signals for driving switching elements that constitute an inverter are often generated by a PWM method. The PWM method is a method to appropriately control the ratio between ON-time and OFF-time in a switching period that is a specific period of time in order to output a voltage that can be instantaneously output by the switching elements, that is, a voltage between a voltage applied between a collector and an emitter and a zero voltage, as the average voltage in the switching period.
The PWM method includes an asynchronous PWM method and a synchronous PWM method. A synchronous PWM signal for a switching element in the PWM method is generated by a carrier-wave comparison method to compare the magnitude of an intended output-voltage command value and a carrier wave. In the following descriptions, the intended output-voltage command value is referred to as “output-voltage command value” and the carrier wave is referred to as “carrier”.
The synchronous PWM method is a method to control the carrier frequency so as to become an integer multiple of the frequency of an output-voltage command value, that is, to control the carrier frequency on the basis of the frequency of the output-voltage command value. The asynchronous PWM method is a method used when the carrier frequency is fixed at any frequency of the output-voltage command value.
The frequency of the output-voltage command value varies on the basis of the load state. In particular, in a case where a load of an inverter is a motor, as the rotation speed of the motor is increased, the frequency of the output-voltage command value becomes higher. Patent Literature 1 discloses an inverter device to which direct-current power is input. By using the PWM method, the inverter device applies alternating-current power of a given voltage and frequency to a motor to drive the motor at variable speeds.